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Composition
Dragendroff's reagent is a solution of potassium bismuth iodide prepared from basic bismuth nitrate (Bi(NO3)3), tartaric acid, and potassium iodide (KI).

Preparation
Dragendorff's reagent is prepared by mixing a concentrated solution of potassium iodide with a solution of bismuth subnitrate in a diluted acid (acetic acid or tartaric acid, hydrochloric acid or sulfuric acid also rarely being used) as a low pH is mandatory for this reagent.

The formation is as follows: The black precipitate of bismuth iodide is formed from the reaction of bismuth ion and potassium iodide.
 * + 3 →  + 3

Then, the reaction between bismuth ion and excess potassium iodide will produce a soluble complex of potassium tetraiodobismuthate which has an orange color.
 * + → K(BiI4)

Reaction
Most of the alkaloids have a tertiary amine group, so the explanation for color reaction could be the following:


 * R3N + HX → [R3NH]+ + X
 * (X = anions of acid)

Then the insoluble complex salt is formed from the reaction between ammonium salt and potassium tetraiodobismuthate.
 * [R3NH]+X- + K[BiI4] → [R3NH]+[BiI4]- + KX

This ion pair has different colors: yellow, orange, red, and brown, which depend on the nature of alkaloids.